This invention relates to a mobile communication system in which a radio speech channel is switched between radio zone areas while keeping the communication of a mobile station.
The mobile communication system comprises, as is well known, a plurality of base stations each having an independent radio zone area, a plurality of mobile switching centers which accommodate the base stations through wire speech path, and a mobile station connectable through a radio speech channel to any one of the base stations. With this mobile communication system the mobile station can move across the radio zone areas during communication to communicate with a telephone subscriber.
In such a mobile communication system, where the mobile station moves to a new radio zone area accommodated by a different mobile switching centers during a conversation, and a situation happens in which the conversation cannot be continued through the radio speech channel used up to now, it is needed to change the radio speech channel in use to a new radio speech channel for the new base station while continuing the conversation.
With reference to FIG. 1, an explanation is made of a handover used in mobile switching centers used in a conventional mobile communication system. This is described in a paper entitled "Nation-wide automobile telephone service using new tracking exchange technology," by T. Goto and T. Eto, which was published in International Switching Symposium '84, Session 32B-4, Florence, 1984.
When a mobile station 1 moves from a radio zone area 12 to a radio zone area 13 during communication with a stationary telephone 6 in a public switched telephone network through radio speech channel 7, an idle radio speech channel 9 in radio zone area 13, and wire speech path 10 and 11 are determined. Specifically, wire speech path 11 is set up in mobile switching center 4, while wire speech path 10 is set up in mobile switching center 5. Mobile switching center 4 instructs mobile station 1 through base station 2 used up to now to switch its radio speech channel (frequency) 7 to a radio speech channel (frequency) 9 assigned to base station 3. After the channel switching the instructed mobile station 1 relays that the radio speech channel has been switched to mobile switching center 4 via base station 3, wire speech path 10, mobile switching center 5, and wire speech path 11. Consequently mobile switching center 4 switches the connection path for a subscriber's wire speech path 14 of stationary telephone 6 from wire speech path 8 to wire speech path 11 which is connected to the new base station 3, wire speech path 10, and mobile switching center 5. And then, mobile switching center 4 releases wire speech path 8 connected between it and the old base station 2.
It is to be noted that, in the above described system, mobile station 1 may be applied to, for example, an automobile, ship, and airplane, if they have a telephone installed.
In the case of the system for effecting handover as described above, however, at a time when the switching is accomplished from the old radio speech channel 7 to the new radio speech channel 9, wire speech path 14 connected to telephone 6 remains connected in mobile switching center 4 to base station 2 via wire speech path 8. This means that the communication path between mobile station 1 and stationary telephone 6 is cut off. Thus, the conversation will be cut off until the connection procedure of subscriber's wire speech path 14 from wire speech path 8 to wire speech path 11 is completed in mobile switching center 4.
Further, the conversation cut-off time, which lasts from the time mobile station 1 switches its channel up to the time the wire speech path is switched in mobile switching center 4, involves a signal transmission delay time between the mobile switching centers. Thus, where a common control signaling system is used between the mobile switching centers the conversation cut-off time will depend on the traffic on control signal lines. In the worst case it is inevitable that the cut-off time becomes extremely long.